This invention relates to fluorescent light sources and, more particularly, to capacitively coupled compact fluorescent light sources wherein at least one of the power coupling conductors is a conductive coating disposed on an external surface of the lamp envelope.
The incandescent lamp has been widely used, especially in interior lighting applications. While simple and inexpensive, the incandescent lamp has very low efficacies, typically producing 15 to 20 lumens per watt of electrical power. The operating life of the incandescent lamp is relatively short and unpredictable. The fluorescent lamp, by contrast, exhibits a very long life and a high efficacy, typically 80 lumens per watt of electrical power. Fluorescent sources have been optimized for overhead lighting in the form of straight or circular tubes which are not well adapted to many lighting needs presently met by the incandescent lamp. While conventional electroded fluorescent lamps provide long life and high efficiency, they require large heavy, and expensive ballasting circuits for operation at line frequencies. An additional problem as one attempts to make small fluorescent lamps is that power losses connected with the electrodes become an increasingly large fraction of the applied power.
In the past, inductive coupling has been used to transfer high frequency electromagnetic power to a low pressure discharge containing a noble gas and mercury vapor. The discharge generates ultraviolet light which is converted to visible light by a phosphor coating on the lamp envelope. Inductive coupling generally utlizes a coil to generate within its volume and the surrounding region an alternating magnetic field and an associated electric field, the latter field lines generally defining a closed path within the conductive plasma discharge. In effect, the current flow within the discharge is such as to form a secondary current in relationship to the driving coil similar to the relationship between the secondary and primary windings of a transformer. Due to collisions, the secondary current in the plasma discharge is somewhat resistive and therefore lossy, part of the loss being converted to light. While the generation of light can be most efficiently accomplished by a uniform excitation of the plasma, the development of closed secondary current paths in the plasma results in non-uniform excitation. Therefore, inductive coupling is not an optimal method for light generation.
Electrodeless fluorescent light sources utilizing inductive coupling have been disclosed in various U.S. Patents. A closed loop magnetic core transformer, contained within a re-entrant cavity in the lamp envelope, induces a discharge in an electrodeless fluorescent lamp in U.S. Pat. No. 4,005,330 issued Jan. 25, 1977 to Glascock et al. Discharge is induced by a magnetic core coil within the envelope of an electrodeless fluorescent lamp in the light source disclosed in U.S. Pat. No. 4,017,764 issued Apr. 12, 1977 to Anderson. In both of the above-mentioned patents the operating frequency is limited to about 50 KHz because of the lossy nature of magnetic materials at high frequency. An electrodeless fluorescent light source utilizing an air-core coil for inductive coupling at a frequency of about 4 MHz is disclosed in U.S. Pat. No. 4,010,400 issued Mar. 1, 1977 to Hollister. However, such a light source has a tendency to radiate power at the frequency of operation and exhibits non-uniform plasma excitation as described hereinabove.
An electrodeless fluorescent light source, utilizing frequencies in the 100 MHz to 300 GHz range, was disclosed by Haugsjaa et al in U.S. Pat. No. 4,189,661, issued Feb. 19, 1980 and assigned to the assignee of the present invention. High frequency power, typically at 915 MHz, is coupled to an ultraviolet-producing low pressure discharge in a phosphor-coated electrodeless lamp which acts as a termination load within a termination fixture.
By contrast to inductive coupling, the excitation of a plasma by capacitive coupling produces a stable and uniform plasma, a condition conducive to maximal light generation. In this case, the electric field lines of the applied oscillatory electromagnetic signal originate on one external electrode, pass through the envelope containing the discharge and terminate on a second external electrode. No closed current paths exist within the plasma in contrast to the situation occurring in inductively coupled plasma discharges described hereinabove.
Capacitive coupling of an electromagnetic pulse to a low pressure discharge in an elongated laser discharge tube is disclosed by Proud et al in pending U.S. application Ser. No. 20,576 filed Mar. 15, 1979 and assigned to the assignee of the present invention. External electrodes are coupled to end portions of the laser discharge tube. The generation of a light emitting, low pressure discharge in a resonant device including an inner electrode and a coaxial outer electrode is disclosed in U.S. Pat. No. 4,063,132 issued Dec. 13, 1977 to Proud et al. The resonant cavity between the electrodes is occupied in part by an annular electrodeless lamp. Repetitive bursts of high frequency oscillations occurring within the cavity are capacitively coupled to a discharge within the electrodeless lamp.
Electrodeless fluorescent light sources typically have a pear-shaped lamp envelope with a re-entrant cavity extending into the lamp envelope for coupling of high frequency power to the discharge. The re-entrant cavity contains a coil in the case of inductive coupling or a solid or hollow electrode in the case of capacitive coupling. While solid or hollow electrodes are generally satisfactory, they have certain disadvantages. Since the electrodes must be inserted into the re-entrant cavity, electrode and re-entrant cavity shapes other than cylindrical are impractical. With a cylindrical re-entrant cavity and a pear-shaped outer envelope, the distance between the re-entrant cavity and the outer surface is non-uniform and the well known phenomenon of self-trapping of emitted radiation can occur. Furthermore, solid or hollow electrodes add appreciable mass to the light source.